- Email: [email protected]
New fluorescent emitter for efficient deep-blue electroluminescence with unusual roll-up character
Materials Letters 158 (2015) 392–394
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
New fluorescent emitter for efficient deep-blue electroluminescence with unusual roll-up character Jun Ye n, Lulu Fu, Huan Liu, Chen Chang, Chi Zhang China-Australia Joint Research Centre for Functional Molecular Materials, School of Chemical & Material Engineering, Jiangnan University, Wuxi 214122, China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2015 Accepted 14 June 2015 Available online 17 June 2015
New deep-blue fluorophore NPAPPS has been synthesized. The compound has both high Td of 452 °C and high Tg above 130 °C, indicative of excellent thermal stability. Photophysical measurements revealed the ICT characteristic both in the ground state and the excited state of NPAPSS, as well as its high Φf, 0.92 in cyclohexane and 0.71 in CH2Cl2. NPAPSS also has a bipolar nature reflected from the oxidation and reduction responses in CV tests. When used as an emitter in the non-doped electroluminescence device, NPAPPS exhibited an unusual roll-up character. At 20, 60, 100 mA cm-2, the device gave efficient current efficiencies of 3.80, 3.97, 4.05 cd A 1 respectively. This roll-up character suggests that NPAPSS is a practically favorable fluorescent emitter. & 2015 Elsevier B.V. All rights reserved.
Keywords: OLED Deep-blue fluorophore Roll-up Optical materials and properties Luminescence
1. Introduction Organic light-emitting diodes (OLEDs) have attracted much attention for their application in full-color display and solid-state lighting. The blue emission has been especially concerned over the past few decades due to its irreplaceable role in generating white emission and saving the power consumption of the display (smaller CIE y-value is preferred) [1,2]. Accordingly, a large number of efficient deep-blue fluorophores with CIE y o0.15 has been rapidly developed [3–8]. Significantly, the electro-fluorescence efficiency can even be equal to that of electro-phosphorescence when purely organic molecules that can give efficient thermally activated delayed fluorescence (TADF) through up-conversion of triplet to singlet states are used [9,10]. However, those efficient fluorophores mostly suffer from obvious efficiency roll-offs at some practical current density (e.g. 20 mA cm 2) or luminescence (e.g. 1000 cd m 2). This is mainly caused by exciton-related actions at high current densities like singlet–singlet annihilation and singlet–polaron quenching. Especially, the roll-off will become more severe for TADF materials since the long life-time triplet excitons are involved. Therefore, finding an efficient deep-blue fluorophore with negligible efficiency roll-off is practically significant but still a hard task. In this work, we report a new efficient deep-blue fluorophore, di(4-(4-(naphthalen-1-yl(phenyl)amino) phenyl)phenyl)sulfone (NPAPPS), which unusually exhibited a gradual increase in current efficiency with the current density n
Corresponding author. Fax: þ86 510 85917763. E-mail address: [email protected] (J. Ye).
http://dx.doi.org/10.1016/j.matlet.2015.06.054 0167-577X/& 2015 Elsevier B.V. All rights reserved.
going up to 100 mA cm 2 (4200 cd m 2).
2. Experimental 2.1. General information 1
H NMR spectra and mass spectra were measured respectively with a Varian Gemin-400 Varian spectrometer and a Finnigan 4021C GC–MS spectrometer. Differential scanning calorimetry (DSC) was performed using a Perkin-Elmer Pyris DSC 6 instrument operated at a heating rate of 10 °C min 1 in a nitrogen atmosphere. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken using a TA SDT Q600 instrument under a nitrogen atmosphere at a heating rate of 20 °C min 1. The absorption and emission spectra were recorded on a Hitachi U-3010 UV–vis spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. Cyclic voltammetry was performed with a CHI600A analyzer with the scan rate of 100 mV s 1 at room temperature, with a glassy carbon working electrode, a Pt wise auxiliary electrode, and an aqueous saturated calomel electrode (SCE) as the reference electrode. Tetra-nbutylammoniumhexafluorophosphate (TBAPF6, 0.10 M) was used as the electrolyte and CH2Cl2 as the solvent, respectively. The ferrocene/ferrocenium couple was used as the internal standard.
J. Ye et al. / Materials Letters 158 (2015) 392–394
393 O
O Br
S
S
O
O Br
(HO)2B
N
N
N
NPAPPS Scheme 1. Synthesis and structure of NPAPPS.
2.2. Synthesis of NPAPPS NPAPPS was synthesized by palladium(0)-catalyzed Suzuki cross-coupling reaction between (4-(naphthalen-1-yl(phenyl) amino)phenyl)boronic acid and 4,4'-dibromodiphenylsulfone, as shown in the Scheme 1. Toluene (6 ml), ethanol (2 ml), and 2 M aqueous Na2CO3 (4 ml) were added to a mixture of di(4-bromophenyl)sulfone (374 mg, 1 mmol), 4-(naphthalen-1-yl(phenyl) amino)phenylboronic acid (850 mg, 2.5 mmol) and Pd(PPh3)4 (117 mg, 0.1 mmol). With stirring, the suspension was heated at 90 °C for 24 h under a nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel (CH2Cl2/petroleum ether (2:1)) to give a light yellow solid (515 mg, 64%). 1H NMR (400 MHz, CDCl3) δ [ppm]: 7.96–7.88 (m, 8H), 7.80 (d, J ¼8.2 Hz, 2H), 7.63 (d, J ¼8.5 Hz, 4H), 7.52–7.44 (m, 4H), 7.40–7.34 (m, 8H), 7.23 (t, J ¼7.4 Hz, 4H), 7.12 (d, J¼ 7.6 Hz, 4H), 7.03–6.97 (m, 6H). EI MS (m/z): 804.28.
Fig. 1. TGA curve (inset: DSC curve) of NPAPPS.
2.3. OLED fabrication and measurement ITO coated glass with a sheet resistance of 15 Ω square 1 was used as the substrate. Before device fabrication, the ITO glass substrates were cleaned with isopropyl alcohol and deionized water, dried in an oven at 120 °C, treated with UV-ozone, and finally transferred to a vacuum deposition system with a base pressure better than 1 10 6 Torr for organic and metal deposition. The devices were fabricated by evaporating organic layers with an evaporation rate of 1–2 Å s 1. The cathode was completed through thermal deposition of LiF at a deposition rate of 0.1 Å s 1, and then capped with Al metal through thermal evaporation at a rate of 10 Å s 1. The overlap between ITO and Al electrodes was 3.3 3.3 mm2 as the active emissive area of devices. EL luminescence, spectra and CIE color coordinates were measured with a Spectrascan PR650 photometer and the current–voltage characteristics were measured with a computer-controlled Keithley 2400 Source Meter under ambient atmosphere.
3. Result and discussion 3.1. Thermal properties Fig.1 shows the TGA and DSC curves, which respectively revealed high decomposition temperature Td (corresponding to 5% weight loss) of 452 °C and high glass transition temperature (Tg) above 130 °C. Such high values would benefit to stabilize NPAPPS itself under thermal evaporation treatment as well as its film morphology during device operation. 3.2. Optical properties Photophysical properties of NPAPPS were analyzed using ultraviolet–visible and photoluminescence spectrometers. Fig. 2 shows the spectra in nonpolar cyclohexane and polar CH2Cl2. The absorption bands in the region from 260 to 290 nm can be
Fig. 2. Normalized absorption and emission spectra of NPAPPS in cyclohexane and CH2Cl2.
attributed to the π–π* transition of the naphthalenyl units, while the broad and strong absorption peaks at 354 nm (cyclohexane) or 364 nm (CH2Cl2) are assigned to the intramolecular charge transfer (ICT) transition between the electron-donating N-phenylnaphthalen-1-amine moiety and the electron-accepting diphenylsulfone moiety. Such a distinct shift of 10 nm from cyclohexane to CH2Cl2 indicates that there should be a dipole moment involved in the ground state of NPAPPS. And a larger dipole moment can be demonstrated in the excited state as evident from the significant fluorescence shift, that is, from 405 nm in cyclohexane to 471 nm in CH2Cl2. This ICT-characteristic emission endows NPAPPS with high fluorescent quantum yields (Φf), 0.92 in cyclohexane and 0.71 in CH2Cl2 respectively, determined as referenced to quinine sulfate (Φf ¼ 0.56 in 1 N H2SO4) [11].
394
J. Ye et al. / Materials Letters 158 (2015) 392–394
OLED: ITO/NPB (50 nm)/TCTA (10 nm)/NPAPPS (30 nm)/SPPO13 (40 nm)/LiF (1 nm)/Al. The device exhibits a deep-blue emission with a peak of 455 nm and CIE coordinates of (0.146, 0.127) at 1000 cd m 2. The EL spectra show little change under different driving voltages. Generally, the performance of EL devices often has a roll-off at high current densities owing to the various exciton quenching processes. However, in this work, we observed a slow increase in current efficiency, which is durative up to the high current density of 100 mA cm 2. As plotted in Fig. 4, the current efficiencies are 3.73, 3.80, 3.91, 3.97, 3.99, 4.05 cd A 1 at 10, 20, 40, 60, 80, 100 mA cm 2, respectively. The maximum current efficiency is unavailable for the limited testing scope of luminescence in our measuring system. The absence of efficiency roll-off should be beneficial for practical application and the reasons are now under investigation.
4. Conclusion Fig. 3. Cyclic voltammogram (Inset: Calculated HOMO/LUMO distributions) of NPAPPS.
A new deep-blue fluorophore NPAPPS has been conveniently synthesized by Suzuki cross-coupling reaction. NPAPPS was then demonstrated with excellent thermal stability (Td of 452 °C; Tg above 130 °C) and high Φf (0.92 in cyclohexane; 0.71 in CH2Cl2). The oxidation and reduction processes in CV test also revealed the bipolar nature of NPAPPS. NPAPPS exhibited an unusual EL performance with the current efficiency slowly increasing when the current density was going up. At 100 mA cm 2, the blue device gave an efficient current efficiency of 4.05 cd A 1. These observations suggested that the novel roll-off-resistant deep-blue fluorophore NPAPPS is a special but practically favored emitter for OLED application.
Acknowledgment This work was supported by Programme of Introducing Talents of Discipline to Universities (111 Project B13025), and the Fundamental Research Funds for the Central Universities (JUSRP11519). Fig. 4. Current efficiency–current density curve of the deep-blue device based on NPAPPS.
3.3. Electrochemical properties Cyclic voltammetry (CV) was conducted to explore the electrochemical properties of NPAPPS. As clearly revealed in Fig. 3, the new compound undergoes both quasi-reversible oxidation and reduction processes, indicating a bipolar nature. Relative to the ferrocene/ferrocenium (Fc/Fc þ ) reference ( 4.8 eV from the vacuum level), the HOMO and LUMO levels of NPAPPS are determined from the onset potentials for oxidation and reduction, respectively, to be 5.26 and 2.55 eV. To gain insight into the electronic structure, a DFT calculation was performed at the B3LYP/ 6-31G (d) level. Fig. 3 also reveals a separation of the HOMO and the LUMO which are dominantly located on the N-phenylnaphthalen-1-amine moiety and the diphenylsulfone moiety respectively, which is a typical feature for D-A structured molecules and might benefit the transport of both holes and electrons. 3.4. Electroluminescence NPAPPS was used as the emitter in a non-doped fluorescent
References [1] M.R. Zhu, C.L. Yang, Chem. Soc. Rev. 42 (2013) 4963–4976. [2] Y.J. Tung, T. Nago, M. Hack, J. Brown, N. Koide, Y. Nagara, Y. Kato, H. Ito, Digest of Technical Papers Society for Inormation Display International Symposium, 35, 2004, pp. 48 –51. [3] C.J. Zheng, W.M. Zhao, Z.Q. Wang, D. Huang, J. Ye, X.M. Ou, X.H. Zhang, C.S. Lee, S.T.J. Lee, Mater. Chem. 20 (2010) 1560–1566. [4] S.L. Tao, Y.C. Zhou, C.S. Lee, X.H. Zhang, S.T. Lee, Chem. Mater. 22 (2010) 2138–2141. [5] K.H. Lee, J.K. Park, J.H. Seo, S.W. Park, Y.S. Kim, Y.K. Kim, S.S. Yoon, J. Mater. Chem. 21 (2011) 13640–13648. [6] J. Huang, N. Sun, Y.Q. Dong, R.L. Tang, P. Lu, P. Cai, Q.Q. Li, D.G. Ma, J.G. Qin, Z. Li, Adv. Func. Mater. 23 (2013) 2329–2337. [7] Y. Zou, J.H. Zou, T.L. Ye, H. Li, C.L. Yang, H.B. Wu, D.G. Ma, J.G. Qin, Y. Cao, Adv. Func. Mater. 23 (2013) 1781–1788. [8] Z.F. Li, B. Jiao, Z.X. Wu, P. Liu, L. Ma, X.L. Lei, D.D. Wang, G.J. Zhou, H.M. Hu, X. Hou, J. Mater. Chem. C 1 (2013) 2183–2192. [9] Q.S. Zhang, B. Li, S.P. Huang, H. Nomura, H. Tanaka, C. Adachi, Nat. Photonics 8 (2014) 326–332. [10] X.K. Liu, Z. Chen, C.J. Zheng, C.L. Liu, C.S. Lee, F. Li, X.M. Ou, X.H. Zhang, Adv. Mater. 27 (2015) 2378–2383. [11] D.F. Eaton, Pure Appl. Chem. 60 (1988) 1107.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
New fluorescent emitter for efficient deep-blue electroluminescence with unusual roll-up character Jun Ye n, Lulu Fu, Huan Liu, Chen Chang, Chi Zhang China-Australia Joint Research Centre for Functional Molecular Materials, School of Chemical & Material Engineering, Jiangnan University, Wuxi 214122, China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2015 Accepted 14 June 2015 Available online 17 June 2015
New deep-blue fluorophore NPAPPS has been synthesized. The compound has both high Td of 452 °C and high Tg above 130 °C, indicative of excellent thermal stability. Photophysical measurements revealed the ICT characteristic both in the ground state and the excited state of NPAPSS, as well as its high Φf, 0.92 in cyclohexane and 0.71 in CH2Cl2. NPAPSS also has a bipolar nature reflected from the oxidation and reduction responses in CV tests. When used as an emitter in the non-doped electroluminescence device, NPAPPS exhibited an unusual roll-up character. At 20, 60, 100 mA cm-2, the device gave efficient current efficiencies of 3.80, 3.97, 4.05 cd A 1 respectively. This roll-up character suggests that NPAPSS is a practically favorable fluorescent emitter. & 2015 Elsevier B.V. All rights reserved.
Keywords: OLED Deep-blue fluorophore Roll-up Optical materials and properties Luminescence
1. Introduction Organic light-emitting diodes (OLEDs) have attracted much attention for their application in full-color display and solid-state lighting. The blue emission has been especially concerned over the past few decades due to its irreplaceable role in generating white emission and saving the power consumption of the display (smaller CIE y-value is preferred) [1,2]. Accordingly, a large number of efficient deep-blue fluorophores with CIE y o0.15 has been rapidly developed [3–8]. Significantly, the electro-fluorescence efficiency can even be equal to that of electro-phosphorescence when purely organic molecules that can give efficient thermally activated delayed fluorescence (TADF) through up-conversion of triplet to singlet states are used [9,10]. However, those efficient fluorophores mostly suffer from obvious efficiency roll-offs at some practical current density (e.g. 20 mA cm 2) or luminescence (e.g. 1000 cd m 2). This is mainly caused by exciton-related actions at high current densities like singlet–singlet annihilation and singlet–polaron quenching. Especially, the roll-off will become more severe for TADF materials since the long life-time triplet excitons are involved. Therefore, finding an efficient deep-blue fluorophore with negligible efficiency roll-off is practically significant but still a hard task. In this work, we report a new efficient deep-blue fluorophore, di(4-(4-(naphthalen-1-yl(phenyl)amino) phenyl)phenyl)sulfone (NPAPPS), which unusually exhibited a gradual increase in current efficiency with the current density n
Corresponding author. Fax: þ86 510 85917763. E-mail address: [email protected] (J. Ye).
http://dx.doi.org/10.1016/j.matlet.2015.06.054 0167-577X/& 2015 Elsevier B.V. All rights reserved.
going up to 100 mA cm 2 (4200 cd m 2).
2. Experimental 2.1. General information 1
H NMR spectra and mass spectra were measured respectively with a Varian Gemin-400 Varian spectrometer and a Finnigan 4021C GC–MS spectrometer. Differential scanning calorimetry (DSC) was performed using a Perkin-Elmer Pyris DSC 6 instrument operated at a heating rate of 10 °C min 1 in a nitrogen atmosphere. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken using a TA SDT Q600 instrument under a nitrogen atmosphere at a heating rate of 20 °C min 1. The absorption and emission spectra were recorded on a Hitachi U-3010 UV–vis spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. Cyclic voltammetry was performed with a CHI600A analyzer with the scan rate of 100 mV s 1 at room temperature, with a glassy carbon working electrode, a Pt wise auxiliary electrode, and an aqueous saturated calomel electrode (SCE) as the reference electrode. Tetra-nbutylammoniumhexafluorophosphate (TBAPF6, 0.10 M) was used as the electrolyte and CH2Cl2 as the solvent, respectively. The ferrocene/ferrocenium couple was used as the internal standard.
J. Ye et al. / Materials Letters 158 (2015) 392–394
393 O
O Br
S
S
O
O Br
(HO)2B
N
N
N
NPAPPS Scheme 1. Synthesis and structure of NPAPPS.
2.2. Synthesis of NPAPPS NPAPPS was synthesized by palladium(0)-catalyzed Suzuki cross-coupling reaction between (4-(naphthalen-1-yl(phenyl) amino)phenyl)boronic acid and 4,4'-dibromodiphenylsulfone, as shown in the Scheme 1. Toluene (6 ml), ethanol (2 ml), and 2 M aqueous Na2CO3 (4 ml) were added to a mixture of di(4-bromophenyl)sulfone (374 mg, 1 mmol), 4-(naphthalen-1-yl(phenyl) amino)phenylboronic acid (850 mg, 2.5 mmol) and Pd(PPh3)4 (117 mg, 0.1 mmol). With stirring, the suspension was heated at 90 °C for 24 h under a nitrogen atmosphere. When cooled to room temperature, the mixture was extracted with CH2Cl2 and dried over Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel (CH2Cl2/petroleum ether (2:1)) to give a light yellow solid (515 mg, 64%). 1H NMR (400 MHz, CDCl3) δ [ppm]: 7.96–7.88 (m, 8H), 7.80 (d, J ¼8.2 Hz, 2H), 7.63 (d, J ¼8.5 Hz, 4H), 7.52–7.44 (m, 4H), 7.40–7.34 (m, 8H), 7.23 (t, J ¼7.4 Hz, 4H), 7.12 (d, J¼ 7.6 Hz, 4H), 7.03–6.97 (m, 6H). EI MS (m/z): 804.28.
Fig. 1. TGA curve (inset: DSC curve) of NPAPPS.
2.3. OLED fabrication and measurement ITO coated glass with a sheet resistance of 15 Ω square 1 was used as the substrate. Before device fabrication, the ITO glass substrates were cleaned with isopropyl alcohol and deionized water, dried in an oven at 120 °C, treated with UV-ozone, and finally transferred to a vacuum deposition system with a base pressure better than 1 10 6 Torr for organic and metal deposition. The devices were fabricated by evaporating organic layers with an evaporation rate of 1–2 Å s 1. The cathode was completed through thermal deposition of LiF at a deposition rate of 0.1 Å s 1, and then capped with Al metal through thermal evaporation at a rate of 10 Å s 1. The overlap between ITO and Al electrodes was 3.3 3.3 mm2 as the active emissive area of devices. EL luminescence, spectra and CIE color coordinates were measured with a Spectrascan PR650 photometer and the current–voltage characteristics were measured with a computer-controlled Keithley 2400 Source Meter under ambient atmosphere.
3. Result and discussion 3.1. Thermal properties Fig.1 shows the TGA and DSC curves, which respectively revealed high decomposition temperature Td (corresponding to 5% weight loss) of 452 °C and high glass transition temperature (Tg) above 130 °C. Such high values would benefit to stabilize NPAPPS itself under thermal evaporation treatment as well as its film morphology during device operation. 3.2. Optical properties Photophysical properties of NPAPPS were analyzed using ultraviolet–visible and photoluminescence spectrometers. Fig. 2 shows the spectra in nonpolar cyclohexane and polar CH2Cl2. The absorption bands in the region from 260 to 290 nm can be
Fig. 2. Normalized absorption and emission spectra of NPAPPS in cyclohexane and CH2Cl2.
attributed to the π–π* transition of the naphthalenyl units, while the broad and strong absorption peaks at 354 nm (cyclohexane) or 364 nm (CH2Cl2) are assigned to the intramolecular charge transfer (ICT) transition between the electron-donating N-phenylnaphthalen-1-amine moiety and the electron-accepting diphenylsulfone moiety. Such a distinct shift of 10 nm from cyclohexane to CH2Cl2 indicates that there should be a dipole moment involved in the ground state of NPAPPS. And a larger dipole moment can be demonstrated in the excited state as evident from the significant fluorescence shift, that is, from 405 nm in cyclohexane to 471 nm in CH2Cl2. This ICT-characteristic emission endows NPAPPS with high fluorescent quantum yields (Φf), 0.92 in cyclohexane and 0.71 in CH2Cl2 respectively, determined as referenced to quinine sulfate (Φf ¼ 0.56 in 1 N H2SO4) [11].
394
J. Ye et al. / Materials Letters 158 (2015) 392–394
OLED: ITO/NPB (50 nm)/TCTA (10 nm)/NPAPPS (30 nm)/SPPO13 (40 nm)/LiF (1 nm)/Al. The device exhibits a deep-blue emission with a peak of 455 nm and CIE coordinates of (0.146, 0.127) at 1000 cd m 2. The EL spectra show little change under different driving voltages. Generally, the performance of EL devices often has a roll-off at high current densities owing to the various exciton quenching processes. However, in this work, we observed a slow increase in current efficiency, which is durative up to the high current density of 100 mA cm 2. As plotted in Fig. 4, the current efficiencies are 3.73, 3.80, 3.91, 3.97, 3.99, 4.05 cd A 1 at 10, 20, 40, 60, 80, 100 mA cm 2, respectively. The maximum current efficiency is unavailable for the limited testing scope of luminescence in our measuring system. The absence of efficiency roll-off should be beneficial for practical application and the reasons are now under investigation.
4. Conclusion Fig. 3. Cyclic voltammogram (Inset: Calculated HOMO/LUMO distributions) of NPAPPS.
A new deep-blue fluorophore NPAPPS has been conveniently synthesized by Suzuki cross-coupling reaction. NPAPPS was then demonstrated with excellent thermal stability (Td of 452 °C; Tg above 130 °C) and high Φf (0.92 in cyclohexane; 0.71 in CH2Cl2). The oxidation and reduction processes in CV test also revealed the bipolar nature of NPAPPS. NPAPPS exhibited an unusual EL performance with the current efficiency slowly increasing when the current density was going up. At 100 mA cm 2, the blue device gave an efficient current efficiency of 4.05 cd A 1. These observations suggested that the novel roll-off-resistant deep-blue fluorophore NPAPPS is a special but practically favored emitter for OLED application.
Acknowledgment This work was supported by Programme of Introducing Talents of Discipline to Universities (111 Project B13025), and the Fundamental Research Funds for the Central Universities (JUSRP11519). Fig. 4. Current efficiency–current density curve of the deep-blue device based on NPAPPS.
3.3. Electrochemical properties Cyclic voltammetry (CV) was conducted to explore the electrochemical properties of NPAPPS. As clearly revealed in Fig. 3, the new compound undergoes both quasi-reversible oxidation and reduction processes, indicating a bipolar nature. Relative to the ferrocene/ferrocenium (Fc/Fc þ ) reference ( 4.8 eV from the vacuum level), the HOMO and LUMO levels of NPAPPS are determined from the onset potentials for oxidation and reduction, respectively, to be 5.26 and 2.55 eV. To gain insight into the electronic structure, a DFT calculation was performed at the B3LYP/ 6-31G (d) level. Fig. 3 also reveals a separation of the HOMO and the LUMO which are dominantly located on the N-phenylnaphthalen-1-amine moiety and the diphenylsulfone moiety respectively, which is a typical feature for D-A structured molecules and might benefit the transport of both holes and electrons. 3.4. Electroluminescence NPAPPS was used as the emitter in a non-doped fluorescent
References [1] M.R. Zhu, C.L. Yang, Chem. Soc. Rev. 42 (2013) 4963–4976. [2] Y.J. Tung, T. Nago, M. Hack, J. Brown, N. Koide, Y. Nagara, Y. Kato, H. Ito, Digest of Technical Papers Society for Inormation Display International Symposium, 35, 2004, pp. 48 –51. [3] C.J. Zheng, W.M. Zhao, Z.Q. Wang, D. Huang, J. Ye, X.M. Ou, X.H. Zhang, C.S. Lee, S.T.J. Lee, Mater. Chem. 20 (2010) 1560–1566. [4] S.L. Tao, Y.C. Zhou, C.S. Lee, X.H. Zhang, S.T. Lee, Chem. Mater. 22 (2010) 2138–2141. [5] K.H. Lee, J.K. Park, J.H. Seo, S.W. Park, Y.S. Kim, Y.K. Kim, S.S. Yoon, J. Mater. Chem. 21 (2011) 13640–13648. [6] J. Huang, N. Sun, Y.Q. Dong, R.L. Tang, P. Lu, P. Cai, Q.Q. Li, D.G. Ma, J.G. Qin, Z. Li, Adv. Func. Mater. 23 (2013) 2329–2337. [7] Y. Zou, J.H. Zou, T.L. Ye, H. Li, C.L. Yang, H.B. Wu, D.G. Ma, J.G. Qin, Y. Cao, Adv. Func. Mater. 23 (2013) 1781–1788. [8] Z.F. Li, B. Jiao, Z.X. Wu, P. Liu, L. Ma, X.L. Lei, D.D. Wang, G.J. Zhou, H.M. Hu, X. Hou, J. Mater. Chem. C 1 (2013) 2183–2192. [9] Q.S. Zhang, B. Li, S.P. Huang, H. Nomura, H. Tanaka, C. Adachi, Nat. Photonics 8 (2014) 326–332. [10] X.K. Liu, Z. Chen, C.J. Zheng, C.L. Liu, C.S. Lee, F. Li, X.M. Ou, X.H. Zhang, Adv. Mater. 27 (2015) 2378–2383. [11] D.F. Eaton, Pure Appl. Chem. 60 (1988) 1107.